1. Field of the invention
This invention relates to an optical semiconductor device such as an optical modulator, an optical switch, and the like, which utilizes a nonlinear effect caused by excitons in the semiconductor layers thereof.
2. Description of the prior art
In recent years, optical semiconductor devices such as optical modulators, optical bistable devices, and the like have been extensively developed in order to realize superhigh speed optical communication, optical logic circuits, and the like. Of great interest as a means to achieve these objectives, there can be mentioned a device structure utilizing a quantum-well effect, in which tens to hundreds of two kinds of very thin semiconductor layers with different band gaps are alternately formed into a quantum-well structure. The term quantum-well structure used herein refers to a thin-layer structure composed of alternate layers consisting of first semiconductor layers with a thickness smaller than the de Broglie wavelength of about 200 .ANG. to 300 .ANG. with respect to electrons or holes and second semiconductor layers with a band gap greater than that of the first semiconductor layer. In recent years, epitaxial growth techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc., have been deVeloped, by which such a thin-layer structure can be produced readily.
Because each layer in the multiple quantum-well structure has a very small thickness, electrons and holes in the multiple quantum-well layer cannot move freely in the direction of thickness, so that they have a strong tendency to be confined two-dimensionally in the plane perpendicular to the direction of thickness. Moreover, the bound energy of an exciton into which an electron and a hole are bound together by their Coulombic attraction is increased because of the two-dimensional confinement of electrons and holes, so that excitons occur under heat energy at room temperature. There are proposed several optical semiconductor devices utilizing such excitons present at room temperature, which include an electric field effect optical modular. FIG. 6 shows a sectional view of a conventional electric field effect optical modulator, which is produced as follows: On a (100)-oriented n-GaAs substrate 60, an n-Al.sub.0.3 Ga.sub.0.7 As layer 61, a multiple quantum-well (MQW) layer (composed of alternate layers consisting of fifty undoped GaAs well layers 62 of a thickness of 100 .ANG. each and forty-nine undoped Al.sub.0.3 Ga.sub.O.7 As barrier layers 63 of a thickness of 100 .ANG. each), a p-Al.sub.0.3 Ga.sub. O.7 As layer 64, and a p-GaAs layer 65 are successively grown. Then, by photolithography and chemical etching techniques, the central portion of each of the n-GaAs substrate 60 and the p-GaAs layer 65 is removed into a circular shape with a diameter of 200 .mu.m, resulting in circular windows 66. Next, an n-sided electrode 67 and a p-sided electrode 68 are formed on the back and upper faces of this device other than the circular windows, respectively.
FIG. 7 shows the band edges when an electric field is applied to the above-mentioned optical modulator, and FIG. 8 shows absorption spectra obtained when the optical modulator is irradiated with light through the circular window. When an appropriate forward voltage is applied across the p-n junction of the optical modulator, the multiple quantum well becomes flat as shown in FIG. 7a, and the wave functions of both electrons and holes in the conduction band and the valence band have a maximum value at the center of each of the well layers, so that the transition matrix element represented by the following formula (1) has a large value, resulting in large transition probability: EQU &lt;.PSI..sub.c .vertline.P.vertline..PSI..sub.v &gt; (1)
where .PSI..sub.c and .PSI..sub.v are wave functions of electrons and holes in the first quantum state, respectively, and P is momentum operator. In contrast, when a reversed bias voltage is applied across the p-n junction, the band edges incline as shown in FIG. 7b, and the .PSI..sub.c and the .PSI..sub.v are biased in the opposite directions to each other, so that spacial overlaps between these wave functions become small. Therefore, the matrix element of the formula (1) has a small value, resulting in a reduction of transition probability. At the same time, the quantum states Ec and Ev of electrons and holes shift to the lower energy side. The absorption spectra shown in FIG. 8 reflect such an effect. The absorption spectrum la shown in FIG. 8(a) is obtained in the case where bands are flat as shown in FIG. 7a, and there is a sharp peak E.sub.H at the absorption edge, corresponding to the exciton transition of electrons and heavy holes. On the other hand, when a reversed bias voltage is being applied, the absorption peak E.sub.H shifts to the lower energy side and the height thereof decreases. The second absorption peak E.sub.L that appears in each spectrum corresponds to an exciton transition of electrons and light holes.
The above-mentioned optical modulator with the multiple quantum well is irradiated with light of a wavelength corresponding to energy h.gamma. shown in FIG. 8 through one of the circular windows, and the intensity of light emitted from the other circular window can be modulated by means of an applied voltage. In the situation of FIGS. 7a and 8(a), incident light is almost absorbed into the absorption peak E.sub.H at the band edge, so that the intensity of emitted light becomes small. On the other hand, in the situation of FIGS. 7b and 8(b), the absorption edge peak E.sub.H shifts to the lower energy side and the height thereof decreases, so that the absorbance with respect to the incident light of energy h.gamma. is remarkably decreased, resulting in an increase in the emitted light intensity.
In such an optical modulator, the modulation index of emitted light is determined by the height of the absorption curve on the higher energy side of the peak E.sub.H shown in FIG. 8. There is another absorption peak E.sub.L, which corresponds to an exciton transition of electrons and light holes, on the higher energy side of the peak E.sub.H, so that when a voltage is applied to the optical modulator, incident light of energy h.gamma. is absorbed into the peak E.sub.L. There has been proposed a semiconductor device using the quantum effect of one dimension in which such an influence of the peak E.sub.L is reduced (T. Hayakawa et al. U.S. Pat. application Ser. No. 159,797, U.S. Pat. No. 4,894,836). As an example of this semiconductor device, an optical modulator produced on a (111)-oriented GaAs substrate is disclosed therein, whereas a conventional optical modulator is produced on a (100)-oriented GaAs substrate. FIG. 9 compares photoluminescene excitation spectra of multiple quantum wells above the (100)-oriented and the (111)-oriented substrates at 77K. As seen from this figure, when the (111)-oriented multiple quantum well is used, the energy separation between the peaks E.sub.H and E.sub.L becomes large and the height of the peak E.sub.H is greater than that of the peak E.sub.L. This is due to the anisotropy of the heavy-holes band in the [100] and [111] directions. That is, the effective mass of heavy holes in the [111] direction is greater than in the [100] direction and the energy levels of heavy holes rise only slightly from the bottom of the quantum well, so that the peak E.sub.H shifts to the lower energy side, resulting in an increase in the energy separation between the peaks E.sub.H and E.sub.L. Moreover, this is because the effective mass of heavy holes in the (111) plane is greater than in the (100) plane, so that the state density of heavy holes within the quantum well becomes large, resulting in an increase in transition probability. The use of such an effect makes it possible to increase the height of the absorption curve on the higher energy side of the peak E.sub.H, so that the modulation amplitude of emitted light can be increased.
As a typical example of other conventional optical semiconductor devices, there can be mentioned an optical bistable device utilizing exciton peaks such as a self-electrooptic effect device (SEED) proposed by Miller et al., which is described in detail in the following article: D.A.B. Miller, D.S. Chemla, T.C. Damen, T.H. Wood, C.A. Bvrrus, Tr, A.C. Gossard, and W. Wigmann, "The quantum well self-electrooptic effect device, optoelectronic bistability and oscillation, and self-linearized modulation," IEEE, J. Quantum Electron , vol.Qe-21, pp. 1462(1985).
The operating principle of this optical switch will hereinafter be explained briefly. FIG. 3 shows the optical switch in which the multiple quantum-well device shown in FIG. 6 is connected in series with an external resistor R and a constant reversed bias voltage is applied between both sides of the multiple quantum-well device. When the multiple quantum-well device is irradiated through one of the circular windows with light of photoenergy near the band gap between the band edges at the time when no voltage is applied, an absorption coefficient for incident light at the time when a voltage is applied becomes small because of the Stark effect of the quantum well as shown in FIGS. 7 and 8. Raising the intensity of the incident light increases a photocurrent that arises from the absorption of the incident light, so that while a voltage drop with respect to the external resistor R is increased, a voltage applied to the multiple quantum well is lowered. Therefore, the absorption spectrum of the multiple quantum well at the time when a voltage is applied approaches the absorption spectrum la shown in FIG. 8(a).
When resonance occurs between the energy of incident light and the exciton transition energy of electrons and heavy holes, the absorbance of the multiple quantum well increases, and the amount of emitted light rapidly decreases. Even if the amount of the incident light is lowered under such a condition, optical output power is maintained at a low level because of a large photocurrent that arises from the exciton absorption, resulting in a hysteresis as shown in FIG. 10. In this kind of optical switch, the ON/OFF ratio of bistable output power is determined by the depth of the absorption curve on the higher energy side of the peak E.sub.H shown in FIG. 8. Therefore, a large ON/OFF ratio of the bistable output power can be obtained by use of a (111)-oriented quantum well.